Was having what I thought were FICM problems with a no start. Turned out to be a blown alt, put a new one in with new batteries and fired right up and ran GREAT at idle for about 10 mins. Charging system did not seem to be working though was only reading 12.5v while running?!?! let the truck warm up for 10 mins no missing no smoke was great, shut it off and tried to start it again and had a hard time getting it going and when I did was running REALLY rough, rpm jumped from 500-1000 then it died. Tried to start it again and would fire and kinda go then die would not stay running. Batteries were only reading 10v now and Alternator was really hot and I think its blown again. Almost seems like truck is burning the alternator then running off the batteries and draining them real quick and when you shut it off theres not enough voltage getting to everything now. Ficm was reading 26v at idle at first which is very low never tested it when it was idleing real smooth for the 10 mins. There seems to be something wrong with the charging system and its blowing the alternator right away. Any ideas?

It's possible. Need to get a load meter and, with the alternator unhooked, see what kind of load is being put on the batteries while it is running. If it's not extremely high, then it might just be a bad alternator. What puzzles me is that you should be able to run that engine for hours on batteries alone before it drains them to 10V. If they were fully charged, there had to be some red hot wires somewhere for that much juice to be sucked out of them.

charge the batteries, I would think it could run on battery power only. All the alternator does it charge the batteries right? The batteries wouldnt have drained completely in 10 mins you might have another problem. click on the no start thread in my signature for test procedures. Hopefully something there will help. Good luck

For what it's worth, when diagnosing charging system issues on these trucks, bear in mind that when the engine is first started cold, the glow plugs can remain energized for up to two minutes AFTER the engine is started depending on how cold the ambient temperature is. Since glow plugs draw A LOT of current, you will see decreased voltage readings when they are energized, which can lead you to believe you have a weak alternator.

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